1. Field of the Invention
The invention relates to a method of detecting and processing optical signals from multiple optical waveguides.
2. Art Background
There are many instances where it is necessary to obtain, at a central location, information provided by multiple sensors. As one example, distributed fiber-optic (FO) sensors, which are used in various industrial, medical and environmental monitoring applications, provide information over one or more optical waveguides. It is often required to identify a sensor or a number of sensors where an event has occurred.
When changes in light intensity are used as a recognition signal of a measured event, a variety of different methods are used to identify the individual sensor where the change in intensity had occurred. For example, an optical time domain reflectometer, OTDR, is commonly employed to locate intensity changes in an array of sensors connected in series. J. Burck and E. Sensfelder “Optical fiber sensors for the distributed measurement of hydrocarbons. SPIE, Vol 3540 pp 98-109. Hopenfeld (U.S. Pat. No. 5,200,615) presented a multi-sensor system where the individual sensors are coated with polymers containing fluorescent dyes or phosphor materials to detect and locate leaks. The coatings, emitting light at differing frequencies, reveal the location of the particular sensor, which was exposed to the leaking fluid. Optical pulses with varying width, amplitude and frequency were employed by Spillman, et al (U.S. Pat. No. 4,963,729) to interrogate individual sensors in a multi-sensor system. Maurice et al. disclosed (U.S. Pat. No. 5,408,092) a circuit for the interrogation of individual sensors in a distributed sensors system using a multiplexer which is controlled by a timing device and a logic circuit.
In certain applications, plastic optical fibers are used to both detect substances and transmit light to and from the sensing zone of the fiber. Hopenfeld (U.S. Pat. No. 5,828,798) described such a method where the sensing zone is a small segment of the fiber in a form of a loop. Because of the poor transmission of light through plastic fibers the individual sensors can not be connected in series and therefore the use of an OTDR is not practical. The use of fluorescent dies or phosphor materials in conjunction with plastic fibers is also not practical for long distances because of the weakness of the signal and its large attenuation along the optical fiber.
Interfacing individual sensors with a programmable microcontroller can also be used to identify individual sensors by connecting individual photodetectors to the input/output (I/O) pins of the microcontroller. The use of these systems, however is limited to the number of I/O pins that are readily available. Ganging up several microcontrollers increases the size of the circuit boards, which in turn increases the cost of the enclosure. Microcontroller based systems also require considerable development initially and reprogramming each time the sensor configuration is changed.
It would be desirable to improve upon the above mentioned systems for obtaining information from multiple FO sensors. More generally, it would desirable to improve upon existing methods for obtaining and analyzing information from sensor systems.